Multicast mapping IP to MAC addresses

All multicast IPv4 addresses begin with 1110 in binary leaving 28 bits to define any unique multicast address in IPv4.

All multicast MAC addresses begin with 01:00:5E and have the very next bit set to 0, leaving 23 bits to define any unique multicast MAC addresses.

During encapsulation of a multicast packet, a specific mapping algorithm is used to map the 28-bit multicast IPv4 addresses to the 23 bit multicast MAC addresses. This mapping of a larger address space to a smaller address space means that multiple multicast IP addresses may be mapped to the same multicast MAC address.

By using only specific multicast addresses, engineers can steer clear of multicast addresses that may map to the same MAC address.

Even if an engineer is unaware, it is unlikely that you will have multiple multicast IP addresses mapping to a single MAC address. Not impossible, but unlikely. If this does take place, multicast will still function correctly. As a multicast client decapsulates its received multicast packet, it will see the destination IP address, and will simply discard it if it does not belong to this multicast group. It adds a little overhead, but it doesn't break the multicast operation.